Support feeds with stories with scheme-less urls

Example is https://blog.golang.org/feed.atom.
Urls in this feed are referenced to as scheme-less urls
(//blog.golang.org/context).

This commit adds normalization of story urls using the feed url. If a
story url doesn't have a scheme it will use the scheme of feed's url.
Refactor url normalization in StoryRepository
Handle cases where URI's cannot be parsed when expanding urls
Fix comment typo
Refactor StoryRepository#normalize_url for improved readability
Merge pull request #329 from pascalw/bugfix-schemeless-story-urls

Support feeds with stories with scheme-less urls
Merge pull request #330 from pascalw/bugfix-expand_absolute_urls

Fix feed importing when URI's cannot be parsed when expanding urls